1. Field of the Invention
The invention concerns a method for the measurement of a steady level in a noise-infested signal, and measuring instruments to implement this method.
2. Description of the Prior Art
It is often necessary to measure a steady level in a noise-infested signal. For example, a steady level has to be measured in a signal formed by a voltage that is variable as a function of time, or else the amplitude of a line has to be measured in a spectrum that is variable as a function of time. A prior art method consists in displaying the graph of the signal, with the noise, on the screen of a cathode-ray tube and in visually interpreting the graph so as not to take into account the noise which is represented by a blurred zone having a decreasing luminance on either side of the trace corresponding to the useful signal. This visual interpretation achieves a sort of filtering which takes into account only the central part of the diffused trace appearing on the screen.
This method of visual interpretation is clearly not applicable to an automatic measuring device. In this case, it is common to use a low-pass filter that eliminates frequencies far greater than the fundamental frequency of the signal controlling the vertical deviation in the cathode ray tube. The drawback of this method is its lack of precision, for the measurement is falsified by the remainder of the filtered noise, which gets added to the useful signal, and may be falsified by a non-negligible deformation of the useful signal. The aim of the invention is to overcome this drawback of the prior art.